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From:   "Huang, Ying" <ying.huang@intel.com>
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Liu Shixin <liushixin2@huawei.com>, Michal Hocko <mhocko@suse.com>,
        Yu Zhao <yuzhao@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Sachin Sant <sachinp@linux.ibm.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Kefeng Wang <wangkefeng.wang@huawei.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v10] mm: vmscan: try to reclaim swapcache pages if no
 swap space
In-Reply-To: <CAJD7tkbC=Z6qAE+b6Ch5eVxNY7k0p98i_=RY0m4_3yg5C_zv+A@mail.gmail.com>
        (Yosry Ahmed's message of "Tue, 21 Nov 2023 22:44:32 -0800")
References: <20231121090624.1814733-1-liushixin2@huawei.com>
        <ZVyp5eETLTT0PCYj@tiehlicka>
        <32fe518a-e962-14ae-badc-719390386db9@huawei.com>
        <CAJD7tkbC=Z6qAE+b6Ch5eVxNY7k0p98i_=RY0m4_3yg5C_zv+A@mail.gmail.com>
Date:   Wed, 22 Nov 2023 14:57:24 +0800
Message-ID: <878r6q9sx7.fsf@yhuang6-desk2.ccr.corp.intel.com>
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Yosry Ahmed <yosryahmed@google.com> writes:

> On Tue, Nov 21, 2023 at 10:41=E2=80=AFPM Liu Shixin <liushixin2@huawei.co=
m> wrote:
>>
>>
>> On 2023/11/21 21:00, Michal Hocko wrote:
>> > On Tue 21-11-23 17:06:24, Liu Shixin wrote:
>> >
>> > However, in swapcache_only mode, the scan count still increased when s=
can
>> > non-swapcache pages because there are large number of non-swapcache pa=
ges
>> > and rare swapcache pages in swapcache_only mode, and if the non-swapca=
che
>> > is skipped and do not count, the scan of pages in isolate_lru_folios()=
 can
>> > eventually lead to hung task, just as Sachin reported [2].
>> > I find this paragraph really confusing! I guess what you meant to say =
is
>> > that a real swapcache_only is problematic because it can end up not
>> > making any progress, correct?
>> This paragraph is going to explain why checking swapcache_only after sca=
n +=3D nr_pages;
>> >
>> > AFAIU you have addressed that problem by making swapcache_only anon LRU
>> > specific, right? That would be certainly more robust as you can still
>> > reclaim from file LRUs. I cannot say I like that because swapcache_only
>> > is a bit confusing and I do not think we want to grow more special
>> > purpose reclaim types. Would it be possible/reasonable to instead put
>> > swapcache pages on the file LRU instead?
>> It looks like a good idea, but I'm not sure if it's possible. I can try =
it, is there anything to
>> pay attention to?
>
> I think this might be more intrusive than we think. Every time a page
> is added to or removed from the swap cache, we will need to move it
> between LRUs. All pages on the anon LRU will need to go through the
> file LRU before being reclaimed. I think this might be too big of a
> change to achieve this patch's goal.

We need to identify swap cache pages on file LRU firstly.  It appears
hard from the current definition of page flags.

  /* Filesystems */
  PG_checked =3D PG_owner_priv_1,

  /* SwapBacked */
  PG_swapcache =3D PG_owner_priv_1,       /* Swap page: swp_entry_t in priv=
ate */

--
Best Regards,
Huang, Ying